1. Technical Field of the Invention
This invention relates generally to mixed signal circuitry and more particularly to phase locked loops.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
The local oscillations used in both the transmitter and receiver may be produced by the same or different local oscillation generators. In either case, a local oscillator generator is typically implemented using a fractional N-synthesizer. As is known, a fractional N-synthesizer has a phase lock loop (PLL) topology that allows for fractional adjustments of the feedback oscillation via a feedback fractional N divider. As is also known, the fractional adjustments of the fractional N divider allow for fine tuning of the local oscillation such that, for example, a particular channel may be tuned, a particular intermediate frequency may be achieved, et cetera.
While a fractional-N synthesizer allows for fine-tuning of the local oscillator, its accuracy is limited by the linearity of its components. As is known, a fractional-N synthesizer includes a phase frequency detector, a charge pump, a loop filter, a voltage controlled oscillator and a divider module. Typically, the components that constitute the loop filter have large values and, as such, are large devices if implemented on an integrated circuit. Thus, the loop filter components are generally off-chip, which requires the integrated circuit to include extra pins and requires the corresponding printed circuit board to include extra components.
Therefore, a need exists for an on-chip loop filter that is relatively small with respect to die area and yet provides the desired filtering response.